Aromatic polyamides, which remain soluble in their polymerization mixture, have been synthesized in the 1950's [S. L. Kwolek and H. H. Yang, "History of Aramid Fibers" in "Manmade Fibers: Their Origin and Development", ed. by R. B. Seymour and R. S. Porter, Elsevier Applied Science (1993)]. Further improvements in polymerization techniques via low temperature solution polycondensation were achieved in the late 1960's. This led to the synthesis of para-oriented aromatic polyamides [S. L. Kwolek, U.S. Pat. No. 3,600,350 (1977)]. Several distinctive wholly aromatic polyamides have since been derived from these developments. They include, for example, poly(m-phenylene isophthalamide) (MPD-I), poly(p-benzamide) (PBA), and poly(p-phenylene terephthalamide) (PPD-T). Fibers of MPD-I and PPD-T have been commercialized since 1962 and 1972, respectively, and have been widely adopted for industrial applications.
Aromatic polyamides and their shaped articles are known for their excellent properties such as thermal stability, chemical resistance, dimensional stability, flame resistance, and others. Among these properties, the flame resistance is of particular interest for applications in thermal protective apparel. MPD-I fiber, which provides the combination of excellent thermal stability, flame resistance, and textile-like properties, is especially useful for such applications.
The flame resistance of polymers and shaped articles is measured in terms of the Limiting Oxygen Index (LOI), which is defined as the minimum oxygen content of an oxygen/nitrogen atmosphere needed to support burning. It is well known from literature [e.g. H. H. Yang, "Aromatic High-Strength Fibers", Wiley Interscience, New York (1989)] that wholly aromatic polymers generally exhibit higher LOI values, i.e., better flame resistance, than aliphatic and aliphatic-aromatic polymers. Among aromatic polymers, however, aromatic heterocyclic polymers and aromatic polyamides are superior to aromatic polyamides in flame resistance. For nylon and polyester, polymer modification via additives or polymer blend has been attempted commercially to improve the flame resistance. For aromatic polyamides, Fujie et al. [U.S. Pat. No. 4,196,118 (1980)] taught the use of an organic compound containing phosphorous and halogen as a fire retardant to improve the flame resistance of MPD-I. Fibers of such polymer compositions exhibited excellent flame resistance, but substantially inferior thermal stability.